Breast cancer is the most common form of cancer and the second leading cause of cancer-related deaths amongst women. These deaths will mostly occur due to metastatic burden, demonstrating a pressing need to develop therapies against metastatic progression. The Eya family proteins are over expressed in many cancer types, including breast cancer where they have been shown to be critical for tumorigenesis and metastasis. The Eyas has been shown to regulate these processes through two biochemical functions, as a transcriptional cofactor to the six family of proteins and as a tyrosine phosphatase. Currently, therapies are being developed against both of these functions. The Eyas has another biochemical function as a threonine phosphatase, the activity of which regulates the innate immune response. The role of Eya threonine phosphatase in cancer has yet to be elucidated. However, factors regulating immune response are often over expressed in cancer and can lead to tumor progression and metastasis. These data indicate Eya3, the member of the Eya family with the highest threonine phosphatase activity, may promote breast cancer tumorigenesis and metastasis, leading to insight into new Eya based therapies. Hypothesis: Eya3 threonine phosphatase activity promotes breast cancer metastasis through both tumor cell autonomous roles and non-cell autonomous roles of affecting immune cell behavior. Objective: To utilize both in vitro and in vivo models to assess the role of the Eya3 threonine phosphatase in promoting breast cancer tumorigenesis/metastasis and immune system regulation. Specific Aims: 1) Determine the cell autonomous effects of Eya3 threonine phosphatase on aggressiveness of breast cancer cells. 2) Determine the non-cell- autonomous effects of Eya3 threonine phosphatase on immune cell behavior. 3) Determine the effect of Eya3 threonine phosphatase activity on tumorigenesis and metastasis using in immune competent in vivo mouse models. In vitro models will be utilized to determine Eya3 threonine phosphatase in tumor cell autonomous functions. 66cl4 mouse mammary carcinoma cell lines will be engineered to expressed a phosphatase dead version of Eya3 and proliferation, cell survival, MMP expression/function, migration, and invasion assays will be performed to assess phenotypes. These cells will be used to assess the role of the phosphatase on immune cell modulation by performing in vitro T-cell and macrophage migration assays as well as flow cytometry for changes in immune cell signatures. In vivo mouse models will be used to determine the effect of Eya3 threonine phosphatase on tumorigencity with tumors derived from the 66cl4 cell lines analyzed for size, metastatic ability, and immune cell infiltration by flow cytometry and immunohistochemistry. Research into how the Eya3 threonine phosphatase regulates immune cell behavior and breast cancer tumorigenesis/metastasis can provide insight into treatments for breast cancer patients in addition to other cancer type patients with overexpressed Eya family members.